1. Field of the Invention
The present invention relates to an arithmetic unit for vehicles and a method of operation thereof, and more particularly to an arithmetic unit and method of operation for determining a target amount. The target amount is correlated with two parameters related to the control of an internal combustion engine so that, in a specific region in the vicinity of an area in which the two parameters have a predetermined proportional relationship, the target amount is significantly varied as either of the two parameters is varied. In a region other than the specific region, the target amount is fixed or is varied depending mainly upon only one of the parameters. The present invention also pertains to a throttle valve opening ratio computing device, as an example of the arithmetic unit, for determining a corresponding single-valve opening ratio based on a first throttle valve opening ratio and a second throttle valve opening ratio of a dual-valve type throttle valve.
2. Description of the Related Art
A variety of known arithmetic units for vehicle systems determine a target amount required for control of the vehicle system according to two parameters. In such arithmetic units, a map is prepared and referred to for determining one target amount based on two parameters.
In a dual-valve throttle unit, for example, a target amount (for example, a valve opening ratio of a second throttle valve to be controlled) is determined according to two parameters (for example, a valve opening ratio of a first throttle valve and a corresponding single-valve opening ratio) for the purpose of traction control. FIG. 1 is a graph showing a map representing the correlation of a corresponding single-valve opening ratio TAA with a first throttle valve opening ratio TAM and a second throttle valve opening ratio TAS.
In traction control, when an accelerator pedal is abruptly pressed to increase the first throttle valve opening ratio TAM and realize an excessively large slip rate of wheels, the second throttle valve is closed to reduce the amount of air intake into an internal combustion engine. The corresponding single-valve opening ratio TAA for realizing the appropriate amount of air intake is calculated, and the second throttle valve opening ratio TAS to be controlled is then determined by referring to the map according to the corresponding single-valve opening ratio TAA and the first throttle valve opening ratio TAM actually measured. Such control practically restricts the first throttle valve opening ratio and uses a range FF shown in FIG. 1. In the range FF, the corresponding single-valve opening ratio TAA is substantially determined only by the second throttle valve opening ratio TAS. The structure of the map is thereby relatively simple in this range FF.
In a specific range in the vicinity of an area where the two parameters have a predetermined proportional relationship, for example, a range GG in FIG. 1, the target amount is significantly varied with variation in the two parameters. The map accordingly requires an extremely large data volume for realizing a certain precision.
When the map is referred in the range GG of FIG. 1, dense graduation of the map is required since the target amount is significantly varied with variation in either of the two parameters. Although a residual area other than the specific range GG does not require such dense graduations, a number of map points or map graduations can not be reduced in the residual area since the specific region GG extends around a line of TAM=TAS. When a range requiring dense graduations extends two-dimensionally, the map for determining a target amount according to two parameters should have dense graduations in the whole range.
Such a large map undesirably occupies a large space of a memory unit of a vehicle system where only a limited number of electronic devices are mounted. The large map requires a relatively long time to be referred to for determination of a target amount, thereby preventing quick control.
Some systems for shifting a mean value as an origin of a coordinate system for the purpose of reduction of a map volume storing learned values have been proposed as disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-190138. Such system, however, only reduces the bit number of data by converting absolute data to relative data, and can not reduce the volume of the map used for determining the target amount according to the two parameters in the relationship described above.